Moving coil linear motors

ABSTRACT

A linear motor is described consisting of a central pole piece disposed within a cylindrical pot magnet, the pot magnet having an end plate containing a central aperture into which the central pole piece projects to form an air gap. A magnet shield in the form of a tube is positioned coaxially with and spaced from the central pole piece and a coil is mounted for linear movement in a direction parallel to the axis of the pole piece in the air gap and in the space between the pole piece and the surrounding shield.

Err-721 2842 iJ nited States Patent 1191 1111 3,721,842

Stevenson et al. [451March 20, 1973 54 M VING COIL LINEAR M T R1,390,231 H1965 France ..310/27 inventors: Timothy John S tevensonWindsor; 885,025 l2/196l Great Britam ..3l0/27 Raymond Yardy, Yateley,near OTHER PUBLICATIONS Camberley, both of England Magnetizing CoilArrangement for Linear Actuator, [731 Asslgnw lmemamna' Cmvutm LlmltedrIBM Tech. Disclosure Bulletin, Hu, p. 1803, vol. 12,

London, England NO. H 4/70 [22] Filed: March 9, 1972 Primary ExaminerD.F. Duggan [21] Appl' 233049 Attorney-Hana, Baxley and Spiecens [30]Foreign Application Priority Data [57] ABSTRACT March 18, 1971 GreatBritain ..07,157/7 l A linealmotor is described consisting f a centralpole piece disposed within a cylindrical pot magnet, the pot magnethaving an end plate containing a central apep 58 Field of Search..310/12-14, 27; We mm whch the central pece pmjects 179/1155, 119, 120an air gap. A magnet shield in the form of a tube is positionedcoaxially with and spaced from the central 56] References Cited polepiece and a coil is mounted for linear movement in a direction parallelto the axis of the pole piece in UNITED STATES PATENTS the air gap andin the space between the pole piece 3,505,544 4 1970 Helms ..310 13 andthe surrounding Shield- 3,619,673 11 1971 Helms ..310 13 6 Claims 1Drawing Figure I FOREIGN PATENTS OR APPLICATIONS 789,725 l/1958 GreatBritain ..310 27 MOVING COIL LINEAR MOTORS BACKGROUND OF THE INVENTIONThis invention relates to linear motors.

Linear motors operating on the principle of the moving coil loudspeaker,that is, utilizing the thrust produced by the passage of an electriccurrent through a coil which is mounted in the gap of a magnetic system,are well known. One use of such motors is for the positioning ofmagnetic read/write heads on the discs of a magnetic disc data storagesystem.

The construction of a linear motor suitable for use in a disc storagesystem is shown in U.S. Pat. No. 3,260,870. The design of such motorspresents special problems because of the relatively long stroke, theneed for the best possible efficiency to provide short operating times,and the control of the position of the coil by a servo system. Theserequirements entail using the maximum possible flux density in the gapand at the same time achieving a flux distribution which provides a highdegree oflinearity for the coil thrust.

SUMMARY OF THE INVENTION According to the invention a linear motorincludes a closed magnetic system formed by a central pole piece and asurrounding shell and with an air gap between the central pole piece andthe shell; a magnetic shield within the shell spaced from the centralpole piece and so formed that it shields the central pole piece fromleakage magnetic flux and is maintained close to, or at, magneticsaturation thereby; and a coil mounted for linear movement within theair gap and the space between the shield and the central pole piece.

BRIEF DESCRIPTION OF THE DRAWING The invention will now be described, byway of example, with reference to the accompanying drawing which shows across-sectional view of a linear motor in schematic form.

DESCRIPTION OF THE PREFERRED EMBODIMENT The magnet system consists of acentral pole piece 1 which is secured inside a cylindrical pot magnet 2.A closed system is formed by a magnetic front plate 3, which has acentral hole so that an air gap 4 is formed between the pole piece 1 andthe plate 3.

A magnetic shield tube 5 is mounted coaxially with and spaced from thecentral pole piece 1. A coil 6 is mounted so that it is free to moveparallel to the axis of the pole piece 1 in the air gap 4 and the spacebetween the pole piece and the surrounding shield 5. The application ofa current to the coil 6 produces a magnetic field which interacts withthe magnetic field in the air gaps to cause the coil to move linearly.

The construction of the motor described above is similar to thatdescribed in US. Pat. No. 3,260,870, with the exception of the magneticshield 5. It will be understood that the magnet system may be arrangedin other ways. For example, one or more ring magnets may be securedbetween back and front plates of soft magnetic material. Alternatively,the whole structure may be of soft magnetic material, the requiredmagnetic field being generated by suitable coils wound around one ormore of the magnetic members of the magnet system.

In order to understand the function performed by the shield 5, it isconvenient to consider first the operation of a system without a shield.As has been noted above, it is desirable to use the maximum possibleflux density in the air gap. At the same time, the dimensions of thecentral pole piece 1 are limited by mechanical considerations of thedesign of the coil 6. Consequently, the central pole piece is normallyoperated in a substantially magnetically saturated condition.

Although the system is designed to concentrate the major part of thetotal flux across the air gap, there is inevitably some leakage fluxbetween the inside of the shell, that is, the parts 2 and 3, and thepole piece 1. This leakage flux will be distributed along the length ofthe pole piece 1, so that the maximum flux density in the pole piece 1occurs at the rear end where it abuts the part 2. However, the polepiece is in a magnetically saturated condition, so that the total fluxin the air gap is approximately the idea] gap flux less the leakageflux.

Secondly, the variation of flux along the length of the pole piece 1will cause the force exerted on the coil to vary for a constant currentflowing through the coil. The coil current is usually controlled by aservo system so that magnetic heads attached to an extension of the coilmay be positioned accurately. Any non-linearity in the relationshipbetween the current flowing in the coil and the resulting force on it islikely to result in a degradation of the performance of the servo-loop.

The introduction of the shield 5 of soft magnetic material causes theleakage flux to return through the shield instead of through the centerpole, provided that the leakage flux does not tend to produce a fluxdensity exceeding the saturation flux density of the shield. Hence, theintroduction of the shield largely overcomes the problems of reductionin the flux in the gap 4 and the non-uniformity of the field along thepole piece. However, the shield can itself introduce a problem if it isnot properly designed. As in the case of the center pole piece, themaximum flux in the shield will occur at the end remote from the frontplate. If the shield has a uniform magnetic cross-section, the end ofthe shield adjacent to the front plate must be unsaturated, and ittherefore provides an alternative path for flux between the center poleand the front plate. A substantial proportion of the gap flux can bediverted to this alterna-- tive path when the coil is energized,reducing the effective gap flux. This difficulty can be overcome byensuring that the shield is close to, or at, magnetic saturation at allpoints due to the leakage flux. This can be achieved by suitable shapingof the shield. A satisfactory approximation in one example has beenfound to be three conical sections, indicated at 7, 8 and 9 in thedrawing.

In calculating the dimensions of the shield, if the front plate 3 ismagnetically saturated at the gap region then the shield must be spacedsufficiently far from the front plate to ensure that at any point of thefront plate the sum of the-flux passing to the gap and the leakage fluxto the shield does not exceed the saturation flux density of the frontplate. If the front plate is not saturated in the gap region, theminimum separation of the shield and the front plate is determined bythe maximum acceptable value of fringing field in the region surroundingthe gap. Having determined the length of the shield, the shape of theshield to provide substantially a condition of magnetic saturationthroughout its length may be determined by calculating the distributionof the leakage flux inside the shell. In practice, it may be moreconvenient to estimate the distribution by simple approximation and torefine the results by test and experiment.

It will be appreciated that the requirement for saturation of the shieldmay be achieved wholly, or in part, by varying the magneticcharacteristics of the shield along its length, for example, using twoor more materials with different saturation flux densities or using amaterial which has been processed, say by heat treatment, to provide anon-uniform saturation flux density.

It will also be realized that the linear motor need not be constructedin the form of a pot magnet as described in the embodiment but may, forexample, be in the form of a U-shaped member with a central pole piece.It is common in such structures to attach pole pieces to the ends of thearms to define air gaps between these pole pieces and the central polepiece.

We claim:

1. A linear motor including a central pole piece; a surrounding shelldisposed relative to the central pole piece to form a closed magneticcircuit incorporating an air gap between the central pole piece and theshell; a magnetic shield disposed between the central pole piece and theshell, the shield being spaced from the central pole piece to shield thecentral pole piece from leakage magnetic flux and being maintained closeto, or at, magnetic saturation thereby; and a coil loosely formed aboutthe central pole piece to permit linear movement of the coil in adirection parallel to said central pole piece within the air gap and thespace between the shield and the central pole piece.

2. A linear motor as claimed in claim 1 in which said magnetic shieldhas different parts with different magnetic saturation characteristicsrespectively.

3. A linear motor as claimed in claim 2 in which said magnetic shieldparts are respectively of different thicknesses.

4. A linear motor as claimed in claim 2 in which said magnetic shieldparts are respectively constructed from two or more different materials,the materials having different magnetic saturation characteristicsrespectively.

5. A linear motor as claimed in claim 1 in which the surrounding shellincludes portions substantially parallel to said central pole piece; andplates abutting said portions and extending towards saidpole piece, saidair gap being provided between the plates and the pole piece; therelative dimensions and dispositions of the shield and the plates beingsuch that the shield is spaced sufficiently far from said plates toensure that at any point on the plates the sum of the magnetic fluxpassing to the air gap and the leakage flux to the shield does notexceed the saturation flux density of the plates.

6. A linear motor as claimed in claim 5 including a pot magnet having anopen end and sides formed by a continuous wall and in which saidcontinuous wall forms said portions; and further including a singleannular member abutting the wall at said open end, respectively radiallyopposite parts of said annular member corresponding to said plates; thecentral pole piece being arranged substantially coaxially in the potmagnetand extending into the central aperture of the annular member.

1. A linear motor including a central pole piece; a surrounding shelldisposed relative to the central pole piece to form a closed magneticcircuit incorporating an air gap between the central pole piece and theshell; a magnetic shield disposed between the central pole piece and theshell, the shield being spaced from the central pole piece to shield thecentral pole piece from leakage magnetic flux and being maintained closeto, or at, magnetic saturation thereby; and a coil loosely formed aboutthe central pole piece to permit linear movement of the coil in adirection parallel to said central pole piece within the air gap and thespace between the shield and the central pole piece.
 2. A linear motoras claimed in claim 1 in which said magnetic shield has different partswith different magnetic saturation characteristics respectively.
 3. Alinear motor as claimed in claim 2 in which said magnetic shield partsare respectively of different thicknesses.
 4. A linear motor as claimedin claim 2 in which said magnetic shield parts are respectivelyconstructed from two or more different materials, the materials havingdifferent magnetic saturation characteristics respectively.
 5. A linearmotor as claimed in claim 1 in which the surrounding shell includesportions substantially parallel to said central pole piece; and platesabutting said portions and extending towards said pole piece, said airgap being provided between the plates and the pole piece; the relativedimensions and dispositions of the shield and the plates being such thatthe shield is spaced sufficiently far from said plates to ensure that atany point on the plates the sum of the magnetic flux passing to the airgap and the leakage flux to the shield does not exceed the saturationflux density of the plates.
 6. A linear motor as claimed in claim 5including a pot magnet having an open end and sides formed by acontinuous wall and in which said continuous wall forms said portions;and further including a single annular member abutting the wall at saidopen end, respectively radially opposite parts of said annular membercorresponding to said plates; the central pole piece being arrangedsubstantially coaxially in the pot magnet and extending into the centralaperture of the annular member.